Applying digital methods to the control of systems bears the promise of creating new features, improving performance, providing greater product flexibility, and providing a lower cost. System operating characteristics dictated by a stored program, rather than the parameters of a set of discrete components, can result in cost and space savings as well as capacity for real time adaptation of those characteristics, greater sophistication in control algorithms and the ability to generate, store and recall valuable real-time functional data.
However, digital feedback control requires high resolution and high speed. These requirements have limited the adoption of digital control in many fields. The advent of low cost logic has it made possible the application of digital control techniques to cost sensitive fields. As the cost of digital logic decreases, new opportunities arise.
A typical digitally controlled feedback system has an analog to digital converter, digital loop compensator, power device driver, and an external system to be controlled. An example of a system in which application of digital control can improve performance or lower cost is the switching power supply or DC-to-DC converter. (However, many other systems would also benefit from application of digital control,)
It is very desirable to minimize the cost, size and power dissipation of a low-cost off-line switching power supply for low power applications, such as recharging cells and batteries used in portable consumer appliances, such as entertainment units, personal digital assistants, and cell phones, for example.
A PWM switched power supply requires a variable pulse width that is controlled by an error signal derived by comparing actual output voltage to a precise reference voltage. The pulse width of the switching interval must also be constrained to be within a minimum and maximum duration. These constraints are imposed for correct PWM power supply or motor driver operation.
An example of a digitally controlled system is shown in FIG. 1. In the example shown in FIG. 1, the system is a simple buck DC to DC converter. The fundamental components are the same for any DC to DC converter. The sample system shown in FIG. 1 includes three major components: a compensator preceded by an ADC, PWM and power switches, and passive LC network.
Typically the PWM resolution is required to be much higher than the ADC resolution. If this is not true, the output of the PWM jumps back and forth in code values to satisfy a particular input ADC code. The frequency of this jumping back and forth, which is commonly known as limit cycling, is determined by the control system dynamics. As a result the frequency and size of the ripple can be large. The typical solution to this problem is to increase the PWM resolution. This method can lead to significant complexity in the PWM Design. Techniques such as polyphase clocks or analog methods may be needed to achieve the required resolution. This is particularly true for high speed power supplies which require a very high speed PWM frequency.
There is a need for a digital controller that overcomes the requirements of high resolution and high speed.
There is also a need for a digital controller that can be implemented without expensive multiplication.
There is a further need for a digital controller that has the above characteristics and can have arbitrarily controlled coefficients.
There is also a need for simple, cost effective methods and systems that provide effectively high PWM resolution.